Referring to FIG. 1, in order to enable a driver to more easily control a vehicle, a vehicle steering system is provided with a power assistance function. In the existing market, an ordinary vehicle is usually provided with an electric power steering system 91 (abbreviated as an EPS system).
The main function of a torque detector 92, which is a key component of the EPS system, is to detect the steering torque exerted by the driver when turning a steering wheel 93. Such goal is accomplished by mounting the torque detector 92 on a torsion shaft (not shown) to detect the angular change of the torsion bar so as to generate an output signal corresponding to the angular change.
The torque detecting devices currently available in the market are generally operated under the principle of Hall effect. Referring to FIG. 2, a conventional torque detecting device is equipped with a Hall induction system, and includes a rotor 95, a pair of stators 96 and a Hall element 97. The rotor 95 is fixed in an end of a torque transmitting device (not shown) which has a torsion bar disposed therein, and has an outer ring mounted with a plurality of magnets that have N-S poles arranged one after another. Each of the stators 96 has a plurality of claws, is fixed to the other end of the torque transmitting device, and is made of material with magnetic permeability. The Hall element 97 is fixed to a housing of the conventional torque detecting device for sensing the magnetic flux. Upon rotation of an input shaft relative to an output shaft, the rotor 95 is moved relative to the stators 96, thereby resulting in a change of magnetic flux. In this way, the Hall element 97 can sense changes in magnetic flux so as to determine a torque value.
The hall element 97 can be used to determine the torque value according to the relative angle generated between the input shaft and the output shaft of the torque transmitting device. However, since the rotation of the input shaft relative to the output shaft is relatively small, a high degree of precision is required for the structures of the Hall element 97 and other related elements.
Chinese Patent No. CN101825425B discloses another conventional torque detecting device for sensing a rotational angle of a shaft relative to another shaft. The conventional torque detecting device includes two planet gear units. Each planet gear unit has a sun gear coupled to a respective one of the shafts, and a detecting gear coupled to the sun gear. The rotation of the shaft relative to the another shaft results in relative movement between the sun gears of the two planet gear units. Since the numbers of teeth of the detecting gears of the two planet gear units are different, the detecting gears of the two planet gear units are rotated in different rotational angles. As such, a steering torque can be calculated using the data of the rotational angles and the numbers of teeth of the detecting gears of the two planet gear units.
However, the configuration in which two planet gear units are employed results in a relatively complicated structure and high cost associated with the aforementioned conventional torque detecting device. Moreover, the calculation method in obtaining the steering torque is quite complex.